1. Field of the Invention
The present invention relates to automatic noise normalization and symbol tracking techniques which can be used to enhance the performance of a symbol tracking loop in a QPSK or .pi./4 QPSK digital demodulator.
2. Description of the Background Art
QPSK demodulator symbol tracking loops are generally known. Moreover, the symbol tracking loop of copending application U.S. Ser. No. 08/322,847 includes an error signal normalization feature for extending the dynamic range of the input signal. The demodulator symbol tracking loop of this copending application further includes a fading mitigation feature in which a constant term related to a representation of the absolute power level of the thermal noise floor of the front end components is added to the error signal during normalization to permit the symbol tracking loop to flywheel at a flywheel threshold when the input signal level is unacceptably low. The flywheel threshold is determined based upon the above-noted constant. Accordingly, the fading mitigation feature of the copending application requires that the absolute power level of the thermal noise floor be known in advance. The front end components of the demodulator of the copending application thus must operate with a known or unvarying gain or noise level.
A typical conventional approach includes the implementation of automatic gain control in the front end components to compensate for gain variation. However, automatic gain control undesirably fixes the signal plus noise level, not the noise level itself. The absolute power level of the thermal noise floor therefore cannot be accurately determined and maintained in this typical conventional approach.